A. Field of the Invention
The invention generally concerns photocatalysts that can be used to produce hydrogen from water in a photocatalytic reaction. The photocatalysts include a photoactive material that is capable of absorbing light, which can then excite an electron from the valence band (VB) to the conductive band (CB) and use the excited electron to split water and produce hydrogen. The photoactive material includes a photonic band gap (PBG) that is tuned with or partially overlaps with its electronic band gap (EBG), thereby reducing the likelihood of the excited electron reverting back to its non-excited or ground state and therefore increasing its photocatalytic activity.
B. Description of Related Art
Hydrogen production from water offers enormous potential benefits for the energy sector, the environment, and the chemical industry. While methods currently exist for producing hydrogen from water, many of these methods can be costly, inefficient, or unstable. For instance, photoelectrochemical (PEC) water splitting requires an external bias or voltage and a costly electrode (e.g., Pt-based) for electrolysis of water.
With respect to photocatalytic electrolysis of water from light sources, while many advances have been achieved in this area, most materials are either unstable under realistic water splitting conditions or require considerable amounts of other components (e.g., large amounts of sacrificial hole or electron scavengers) to work, thereby offsetting any gained benefits. By way of example, a semiconductor photocatalyst is a material that can be excited upon receiving energy equal to or higher than its electronic band gap. Upon photo-excitation electrons are transferred from the valence band (VB) to the conduction band (CB) resulting in the formation of an electron (in the CB) and a hole (in the VB). In the case of water splitting, electrons in the CB reduce hydrogen ions to H2 and holes in the VB oxidize oxygen ions to O2. One of the main limitations of most photocatalysts is the fast electron-hole recombination; a process that occurs at the nanosecond scale, while the oxidation-reduction reactions are much slower (microsecond time scale). Over 90% of photo-excited electron-hole pairs disappear before reaction by radiative and non-radiative decay mechanisms (Yamada, et al., 2009).